J . Phys. Chem. 1991, 95,6182-6186
6182
Chemhtry of Large Hydrated Anlon Clusters X - ( H , O ) n , n = 0-59 and X = OH, 0, 02, and Os. 3. Reaction of SOz X. Yang and A. W . Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: February 14, 1991)
The kinetics and mechanisms of reactions of large hydrated anion clusters X-(H20),,,w~grX = 0, OH, 02, and 03,with SO2are studied in a fast flow reactor under well-defined temperatures and pressures. All the clusters, except OH-,react with SO2at near the collision limit; the reactions of the hydrated anions are found to proceed mainly via a ligand switching mechanism where a few water ligands are replaced by one SO2molecule which leads to the formation of stable reaction products. At low temperatures and large cluster sizes, association eventually dominates the reaction mechanism. Interestingly, cm3/s) are found for the reactions between protonated water clusters, H+(H20)#-'* and slow rate constants (k < SO2, which show that different signs of charge on the hydrated clusters lead to different reaction mechanisms, and hence alter the reaction kinetics. The possible applications of the present results to atmospheric ion chemistry and aqueous solution chemistry are also briefly discussed.
Introduction Sulfur dioxide is a very important species in the atmosphere, and it is also well established that it is one of the key trace neutral reactants involved in the negative ion chemistry' of the stratosphere. In fact, the major negative ion species detected in situ is of the form HS0~(H2S04),(HS03)~(HN03)AH20), where a, b, c, d, and e represent the number of the species in the cluster. Moreover, the formation of atmospheric aerosols consisting of about 75% H#04 is believed2to originate mainly from reactions of SOP2 In the lower part of the atmosphere down to the ground, sulfur-containing hydrated ion clusters may in some casts undergo gas-to-particle conversion (reactions leading to aerosol particles) and in other situations become attached to preexisting aerosol^.^.^ in addition, the aqueous chemistry of SO2is also of importance in other branches of science like biology and agriculture and some important industrial processes such as the production of foods, beverages, pharmaceutical, petroleum, and coal, and in the treatment of industry wastes which has also prompted interest in its behavior from an applied point of view. Recently, we successfullyproduced and studied large protonated water clusters H+(H20)n.,d06.7 and negative cluster ions X-(H2O),,+jg (X = OH, 0, 02,and 03)*-'0 for the first time under well-defined temperatures and pressures. Study of the chemistry of large hydrated cluster ions offers an unique opportunity to elucidate the effect of solvation on reactions, thereby contributing to a better understanding of heterogenous atmospheric processes and aqueous solution chemistry at the molecular level. ~~
~~~~
( 1 ) Ferguson, E. E.; Fehsenfeld, F. C.; Albritton, D. L. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.;Academic: New York, 1979; Vol. 1, p 43. Amold, F. In Atmacpheric Chemistry;Goldberg, E. D., Ed.;Springer: Berlin, 1982; p 273. Keeaee, R. G.; Castleman, A. W., Jr. J . Geophys. Res. 1985, 90,5885. (2) Toon, 0. B.; Pollack, J. B. J . Geophys. Res. 1973, 78, 7051. (3) Castleman, A. W., Jr.; Keestc, R. G . Annu. Reu. Earrh Planer. S c f . 1981, 9, 227. (4) Wigley, T. M. L. Nature 1989, 339, 365. (5) Schroeter, L. C. Suljur Dioxide; Pergamon: New York, 1966. (6) Yang, X.; Castleman, A. W., Jr. J . Am. Chem. SOC.1989, I I I, 6845. (7) Yang, X.; Zhang, X.; Castleman, A. W., Jr. Kinetics and Mechanism Studier of the Reactions of Large Protonated Water Clusters H+(H20)nF14 with CHaCN, CH,COCH3, and CH,COOCH, under Thermal Conditions. Inr. J . Mass Spectrom. Ion Processes, in press. (8) Yang, X.; Castleman, A. W., Jr. J . Phys. Chem. 1990, 91, 8500. (9) Yang, X.; Castleman, A. W., Jr. Chemistry of Large Hydrated Anion Clusters X-(H20)n+59, X = OH, 0,02,and 0,.1 . Reaction of C02. J . Am. Chem. Soc., in press. (10) Yang: X.; Zhang, X.; Castleman, A. W., Jr. Chemistr of Large Hydrated Anion Clusters X-(H20)n.ed9, X OH, 0, 02,and 2. Reaction of CH,CN. J . Phys. Chem., in press.
-
8,.
Herein, we present the experimental results of the rate constant measurements and determinations of the mechanisms of the reactions of SO2with X-(H20)n1b59,X = OH, 0, 02,and 03,at several temperatures and at two different pressures. Experimental Section The experiments were performed using a fast flow reactor affixed with a high-pressure ion source and operated Over a range of selected temperatures. The instrument has been described in detail in earlier publi~ations.~J'J~Briefly, OH-(H20),, and O-(H20), anion clusters are formed in the ion source by discharge ionization of a water/helium mixture. Adding a small amount of O2into the source greatly enhances the intensity of 01(H20), and 03-(H20), ions. Thereafter, the clusters are carried by He from the ion source into the flow tube where they react with neutral reactants, SO2in this case, diluted in He and added into the flow tube through a heatable reactant gas inlet.7 Most of the gas in the flow tube is pumped away by a Roots pump, and only a small portion is sampled into the detection chamber where the reaction products and unreacted parent ions are monitored by a quadrupole mass spectrometer and counted with an electron multiplier. Since the reaction time is known through independent pulsing experiments,I3 the slope of a plot of the logarithm of the parent ion intensity versus the neutral reactant concentration yields the reaction rate constant, while mass scanning gives the reaction product distributions. For measurements made at room temperature the experimental ,precision of the rate constant measurements is about f1Wo and the accuracy is estimated to be about 20%. At low temperature, the statistical fluctuations can be as large as 15%, and the deviation from an unknown true value may be as high as 40% due to larger uncertainty in the measurement of pressure, temperature, and ion velocity. Lecture bottle SO2 was obtained from Matherson and used without further purification. Results and Discussion 1. Rate Constant Measurements. The rate constants were measured at three different temperatures, namely 300,200, and 135 K. At room temperature, only clusters comprised of OH-(H20)n+3, @(H20)n-0+ 0,2-(H2O)n-+29 and 03-(H20)n+l (1 1 ) Yang, X.; Castleman, A. W., Jr. Temperature and Cluster Size Dcpendence Studica of Reactions of htonated Water Clusters with Acetonitrile. J . Chem. Phys., in press. (12) Yang, X.; Castleman, A. W., Jr. J . Chem. Phys. 1990, 93, 2405. (13) Upschulte, B. L.; Shul, R. J.; Pasmella, R.; Kccacc, R. 0.;Castleman, A. W., Jr. Inr. J . Mass Spectrom. Ion Processes 1987, 75, 27.
0022-3654/91/2095-6l82$02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 16, 1991 6183
Chemistry of Large Hydrated Anion Clusters
+
-.
TABLE I: Pressure md Clwter Size Dcpcpacaee of tbc Rate Co118traCrOfor the Reactions X-(HZO), S o p Products (T* 300 I() X=OH X I 0 x = 02 X = O3 kuplb kupzC k,d 'kl, kapl kupz k, kl, kupl kxpz k, k l kelp1 kupz k, 2.6 1.6 1.7 1.918 1.8 1.6 1.6 1.714 2.3 1.9 2.0 2.1" 0 0.08 0.15 1 1.8 1.8 2" 1.8 1.7 1.7 1.6I' 1.9 1.5 1.5 1.818 1.6 1.5 1.4 1.7 1.2 1.2 1.718 1.5 1.6 1.5 1.5 1.7l' 2 1.5 1.5 2" 1.4 1.6 3 1.4 1.5 214 1.5 1.2
"
kal 1.6 1 .5
OUnits for all the rate constants are IO4 cm3/s. bkupl = experimentally measured rate constant at p = 0.29 Torr. Ckup2= experimentally measured rate constant at p = 0.56 Torr. d,&.knp= experimental rate constant reported in the literature; the superscript designates the referenas. 'kd = theoretically calculated rate constant using the method presented in ref 21.
a
b
.-a c
2
a
0.B'
I
2
'
I
4
I
'
6
'
I ' I ' I 8 IO 12 Cluster Size n
"
14
'
I
16
'
I
18
I
Figure 1. Dependence of rate constants on cluster size for the reactions of OH-(HIO), with SOz at T = 135 K: ( 0 )experimental value; (solid line) theoretical calculation.
are observed and serve as reactants with SO,. All the measured rate constants are summarized in Table I. The tables also include other values reported1c20 in the literature as well as ones derived from theoretical calculations using a parametrized trajectory method.zi As seen from the comparisons, except for OH-, all the measured rate constants are very close to the collision rates and no pressure dependence is observed. In the case of OH-, a linear pressure dependence is found and the rate constant is measured to be much smaller than the collision rate. This is in accord with the mechanism being one of association, and the third-order rate constant for the reaction of OH- with SOz is determined to be 8 X cm6/s which is reasonably closed to the value 1 .O X 10-26 reported in ref 14 b a d on experiments made with O2as the carrier gas. Large cluster anions do not appear until the temperature is decreased below about 140 K. Table I1 summarizes the rate constants for the clusters with n up to 19 measured at three different temperatures. At the lower temperatures, in the case of all four series of clusters, the measured rate constants are within experiment error of the values calculated for the collision limits. Similar to the observations at room temperature, except for the case of OH-, none of the rate constants display a systematic dependence on pressure. In order to see the trend of the rate constants measured as compared with the theoretical predicted values, the cluster size dependence of the rate constants of the reaction of OH-(HzO), with SOz is plotted in Figure 1. 2. Reaction Mechanisms. OW(HzO)n.OH-, among all the negative ions studied in the present experiments, is the only anion ~~
(14) Fehsenfeld, F. C.; Ferguson, E.
E. J. Chem. Phys. 1974,61, 3181.
(IS) Lindinger, W.;Albritton, D. L.; Fehsenfeld, F. C.; Ferguson,, E.E. J . Chem. Phys. 1975,63, 3238. (16) Fahey, D. W.; Bahringer, H.; Fehsenfeld, F. C.;Ferguson, E. E.J . Chem. Phys. 1982, 76, 1799. (17)Hierl, P. M.;Paulson, J. F. J . Chem. Phys. 1984,80, 4890. ( I 8) Van Doren, J. M.; Barlow, S.E.; DePuy, C.H.; Bierbaum, V. M. J. Am. Chem. Soc. 198'1,109,4412. (19)Morris, R. A.; Viggiano, A. A,; Paulson, J. F. J. Phys. Chem. 1990,
94. 1884. (20) Viggiano, A. A.; Morris, R. A.; Deakyne, C. A.; Dale, F.; Paulson, J. F. J. Phys. Chcm. 1990, 94,8193. (21) Su,T.; Chesnavich, W . J. J. Chem. Phys. 1982, 76, 5183.
I i
5
1'5
;0 Cluater Sire n
I
1
I
5
1'5 Cluaier Size n
Figure 2. Parent (A, top) and product (B, bottom) distribution for the reactions of X-(HzO), with SOz at T = 135 K: (a) OH-(HzO),; (b) O-(HzO)n; (c) Oz-(HzO),; ( 4 03-(H~O)n;(e) HSO~-(HZO),; (f) SO,-(HZO),; (8) H S 0 4 - ( H 2 0 ) ~ and i ; (h) S04-(H~O)ml.
which reacts with SO2via an association mechanism. This observation is the same as that reported by Fehsenfeld and Ferguson.14Hydration changes the reaction mechanism from association to switching: OH-(H20),
+ SO2
+
+
HSO3-(HZO),,, (n - m)(H,O); n 2 1 (1) Solvation has a critical effect on the reaction thermodynamics, and a consideration of the reaction thermodynamics assists in interpreting the reaction mechanismsO2*For the switching re~~
~~
(22)All the thermodynamic data used in this work are cited or derived from the values listed in the following two tables: Lias, S.G.; Bartmw, J. E.;Liebman, J. F.;Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref.Dura 1988, Suppl. No. 1. Keesee, R. G.;Castleman, A. W., Jr. J . Phys. Chem. Ref.Dora 1986, 15, 1011.
6184 The Journal of Physical Chemistry, Vol. 95, No. 16, 1991
Yang and Castleman
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TABLE II: chgtcr Size and Temperature Dcpclldcare. for tbc ReactiosS X-(H@), + So, Pmducta X=OH x=o x = 02 n 300K 200K 135K 300K 200K 135K 300K 200K 135K 0 1.9 2.4 3.1 1.6 2.0 2.2 (2.3) (3.0) (3.5) (2.6) (3.0) (3.6) (1.8) (2.3) (2.8) 1 1.8 1.9 2.6 1.7 2.1 2.5 1.5 1.9 2.0 (1.6) (2.0) (1.8) (2.3) (2.7) (1.9) (2.3) (2.7) (2.4) 2 1.5 1.8 2.4 1.5 1.9 2.4 1.2 1.7 1.8 (1.5) (2.0) (2.4) (1.6) (2.0) (2.4) (1.5) (1.9) (2.3) 3 1.4 1.7 2.3 1.2 1.7 2.3 1.7 1.5 (1.8) (2.1) (1.4) (1.9) (2.2) (1.5) (1.9) (2.2) 4 1.6 2.1 1.6 1.6 2.3 (1.8) (2.1) (2.1) (1.8) (2.1) 5 1.5 1.8 1.5 1.8 1.5 (1.7) (2.1) (1.7) (2.0) (2.0) 6 1.8 1.9 (2.0) (2.0) (1.9) 7 1.9 1.8 (1.9) (1.9) (1.9) 1.7 8 1.8 (1.9) (1.9) (1.9) 9 1.7 1.6 (1.9) (1.9) (1.9) 10 1.6 1.6 (1.9) (1.9) (1.8) 11 1.6 1.6 (1.8) (1.8) (1.8) 12 I .6 1.6 (1.8) (1.8) (1.8) 13 1.5 1.5 (1.8) (1.8) (1.8) 14 1.5 1.5 (1.8) (1.8) (1.8) 15 1.5 (1.8) (1.8) (1.8) 16 1.6 1.5 (1.8) (1.8) (1.8) 1.5 17 1.6 (1.8) (1.8) (1.8) 18 1.5 1.6 (1.8) (1.8) (1.8) 19 1.5 1.5 (1.8) (1.8) (1.8)
x = 03 300K 1.6 (1.6) 1.5 (1.5)
200K 1.9 (2.1) 1.6 (1.9) 1.7 (1.8) 1.7 (1.7)
135K 1.9 (2.5) 1.7 (2.3) 1.5 (2.1) 1.5 (2.1) 1.5 (2.0)
1.5 (2.0) 1.6 ( 1.9) 1.3 (1.9) 1.2 (1.9) 1.7 (1.8)
1.5 (1.8)
'Units for all the rate constants are IO* cm3/s. Values in parentheses are the calculated rate constants. actions, reaction I , the presence of more water ligands bound to OH- anion makes the reaction enthalpy more positive; this is due to the stabilization of OH- by water molecules as a solvent. In the case that all the water ligands are replaced by a SO2molecule OH-(H,O),
+ SO,
-
HS03- + n(H,O)
(2) the enthalpy for reaction 2 is -274.1,-1 12.8,-55.6, 3.6,and 62.8 kJ/mol for n = I , 2,3,4, and 5, respectively. For large hydrated clusters with n > 3, only when the water ligands are merely partially replaced do the reactions become thermodynamically allowed, e.g., OH-(H20),
+ SO,
-
HS03-(H20)+ 3(H20)
+ 46.6 kJ/mol
(3)
This prediction, based on thermodynamic considerations, is clearly verified by observations made in the present experiments. As shown in Figure 2A, the envelope of the parent OH-(H20), cluster distribution reaches an intensity maximum at n = 8. The experimental findings show that the maximum of the reaction product HSO
